diff --git a/CHANGELOG.md b/CHANGELOG.md
index ea688636ef..5868d208c0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,6 +26,7 @@ subset of the arguments, and return a curried type with the remaining ones https
 - Add support for toplevel `await` https://github.com/rescript-lang/rescript-compiler/pull/5940
 - Support optional named arguments without a final unit in uncurried functions https://github.com/rescript-lang/rescript-compiler/pull/5907
 - Add experimental suppport for directives. An annotation such as `@@directive("use client;")` emits `use client;` verbatim before imports https://github.com/rescript-lang/rescript-compiler/pull/5998
+- `genType`: add `Core` standard library support for the following builtin types: `Null.t`, `Nullable.t`, `Undefined.t`, `Dict.t<_>`, `Promise.t<_>`, `Date.t`, `BigInt.t`, `RegExp.t`, `Map.t<_, _>`, `WeakMap.t<_, _>`, `Set<_>`, `WeakSet<_>` https://github.com/rescript-lang/rescript-compiler/pull/6019
 
 #### :boom: Breaking Change
 
@@ -39,6 +40,9 @@ subset of the arguments, and return a curried type with the remaining ones https
 Also, `(. int) => string => bool` is not equivalen to `(. int, string) => bool` anymore.
 These are only breaking changes for unformatted code.
 - Exponentiation operator `**` is now right-associative. `2. ** 3. ** 2.` now compile to `Math.pow(2, Math.pow(3, 2))` and not anymore `Math.pow(Math.pow(2, 3), 2)`. Parentheses can be used to change precedence.
+- `genType`: streamline the treatment of optionals as undefined https://github.com/rescript-lang/rescript-compiler/pull/6022
+  - Represent `option<t>` as `undefined | t` instead of `null | undefined | t`. This is more permissive when importing functions taking optional values (allows to use option types), but stricter when e.g. exporting ReScript functions taking arguments of option type. Fallback: use `Js.undefined<_>` instead.
+  - Represent `{x:option<string>}` as `{x:(undefined | string)}` instead of `{x?: string}`. This is more in line with TS's behaviour. Fallback: use `{x?:string}`.
 
 #### :bug: Bug Fix
 
diff --git a/jscomp/gentype/Converter.ml b/jscomp/gentype/Converter.ml
index a7090466d1..9a7d1c42a2 100644
--- a/jscomp/gentype/Converter.ml
+++ b/jscomp/gentype/Converter.ml
@@ -5,11 +5,9 @@ type t =
   | CircularC of string * t
   | FunctionC of functionC
   | IdentC
-  | NullableC of t
   | ObjectC of fieldsC
   | OptionC of t
   | PromiseC of t
-  | RecordC of fieldsC
   | TupleC of t list
   | VariantC of variantC
 
@@ -68,8 +66,7 @@ let rec toString converter =
       |> String.concat ", ")
     ^ " -> " ^ toString retConverter ^ ")"
   | IdentC -> "id"
-  | NullableC c -> "nullable(" ^ toString c ^ ")"
-  | ObjectC fieldsC | RecordC fieldsC ->
+  | ObjectC fieldsC ->
     let dot =
       match converter with
       | ObjectC _ -> ". "
@@ -120,6 +117,7 @@ let typeGetConverterNormalized ~config ~inline ~lookupId ~typeNameIsInterface
     | Array (t, mutable_) ->
       let tConverter, tNormalized = t |> visit ~visited in
       (ArrayC tConverter, Array (tNormalized, mutable_))
+    | Dict _ -> (IdentC, normalized_)
     | Function
         ({argTypes; componentName; retType; typeVars; uncurried} as function_)
       ->
@@ -189,10 +187,10 @@ let typeGetConverterNormalized ~config ~inline ~lookupId ~typeNameIsInterface
           else (IdentC, normalized_))
     | Null t ->
       let tConverter, tNormalized = t |> visit ~visited in
-      (NullableC tConverter, Null tNormalized)
+      (OptionC tConverter, Null tNormalized)
     | Nullable t ->
       let tConverter, tNormalized = t |> visit ~visited in
-      (NullableC tConverter, Nullable tNormalized)
+      (OptionC tConverter, Nullable tNormalized)
     | Object (closedFlag, fields) ->
       let fieldsConverted =
         fields
@@ -220,26 +218,6 @@ let typeGetConverterNormalized ~config ~inline ~lookupId ~typeNameIsInterface
     | Promise t ->
       let tConverter, tNormalized = t |> visit ~visited in
       (PromiseC tConverter, Promise tNormalized)
-    | Record fields ->
-      let fieldsConverted =
-        fields
-        |> List.map (fun ({type_} as field) -> (field, type_ |> visit ~visited))
-      in
-      ( RecordC
-          (fieldsConverted
-          |> List.map (fun ({nameJS; nameRE; optional}, (converter, _)) ->
-                 {
-                   lblJS = nameJS;
-                   lblRE = nameRE;
-                   c =
-                     (match optional = Mandatory with
-                     | true -> converter
-                     | false -> OptionC converter);
-                 })),
-        Record
-          (fieldsConverted
-          |> List.map (fun (field, (_, tNormalized)) ->
-                 {field with type_ = tNormalized})) )
     | Tuple innerTypes ->
       let innerConversions, normalizedList =
         innerTypes |> List.map (visit ~visited) |> List.split
@@ -379,7 +357,6 @@ let rec converterIsIdentity ~config ~toJS converter =
                 argConverter |> converterIsIdentity ~config ~toJS:(not toJS)
               | GroupConverter _ -> false)
   | IdentC -> true
-  | NullableC c -> c |> converterIsIdentity ~config ~toJS
   | ObjectC fieldsC ->
     fieldsC
     |> List.for_all (fun {lblJS; lblRE; c} ->
@@ -388,9 +365,8 @@ let rec converterIsIdentity ~config ~toJS converter =
            match c with
            | OptionC c1 -> c1 |> converterIsIdentity ~config ~toJS
            | _ -> c |> converterIsIdentity ~config ~toJS)
-  | OptionC c -> if toJS then c |> converterIsIdentity ~config ~toJS else false
+  | OptionC c -> c |> converterIsIdentity ~config ~toJS
   | PromiseC c -> c |> converterIsIdentity ~config ~toJS
-  | RecordC _ -> false
   | TupleC innerTypesC ->
     innerTypesC |> List.for_all (converterIsIdentity ~config ~toJS)
   | VariantC {withPayloads; useVariantTables} ->
@@ -518,13 +494,6 @@ let rec apply ~config ~converter ~indent ~nameGen ~toJS ~variantTables value =
     EmitText.funDef ~bodyArgs ~functionName:componentName ~funParams ~indent
       ~mkBody ~typeVars
   | IdentC -> value
-  | NullableC c ->
-    EmitText.parens
-      [
-        value ^ " == null ? " ^ value ^ " : "
-        ^ (value
-          |> apply ~config ~converter:c ~indent ~nameGen ~toJS ~variantTables);
-      ]
   | ObjectC fieldsC ->
     let simplifyFieldConverted fieldConverter =
       match fieldConverter with
@@ -553,22 +522,12 @@ let rec apply ~config ~converter ~indent ~nameGen ~toJS ~variantTables value =
     in
     "{" ^ fieldValues ^ "}"
   | OptionC c ->
-    if toJS then
-      EmitText.parens
-        [
-          value ^ " == null ? " ^ value ^ " : "
-          ^ (value
-            |> apply ~config ~converter:c ~indent ~nameGen ~toJS ~variantTables
-            );
-        ]
-    else
-      EmitText.parens
-        [
-          value ^ " == null ? undefined : "
-          ^ (value
-            |> apply ~config ~converter:c ~indent ~nameGen ~toJS ~variantTables
-            );
-        ]
+    EmitText.parens
+      [
+        value ^ " == null ? " ^ value ^ " : "
+        ^ (value
+          |> apply ~config ~converter:c ~indent ~nameGen ~toJS ~variantTables);
+      ]
   | PromiseC c ->
     let x = "$promise" |> EmitText.name ~nameGen in
     value ^ ".then(function _element("
@@ -576,39 +535,6 @@ let rec apply ~config ~converter ~indent ~nameGen ~toJS ~variantTables value =
     ^ ") { return "
     ^ (x |> apply ~config ~converter:c ~indent ~nameGen ~toJS ~variantTables)
     ^ "})"
-  | RecordC fieldsC ->
-    let simplifyFieldConverted fieldConverter =
-      match fieldConverter with
-      | OptionC converter1 when converter1 |> converterIsIdentity ~config ~toJS
-        ->
-        IdentC
-      | _ -> fieldConverter
-    in
-    if toJS then
-      let fieldValues =
-        fieldsC
-        |> List.mapi (fun index {lblJS; c = fieldConverter} ->
-               lblJS ^ ":"
-               ^ (value
-                 |> EmitText.arrayAccess ~index
-                 |> apply ~config
-                      ~converter:(fieldConverter |> simplifyFieldConverted)
-                      ~indent ~nameGen ~toJS ~variantTables))
-        |> String.concat ", "
-      in
-      "{" ^ fieldValues ^ "}"
-    else
-      let fieldValues =
-        fieldsC
-        |> List.map (fun {lblJS; c = fieldConverter} ->
-               value
-               |> EmitText.fieldAccess ~label:lblJS
-               |> apply ~config
-                    ~converter:(fieldConverter |> simplifyFieldConverted)
-                    ~indent ~nameGen ~toJS ~variantTables)
-        |> String.concat ", "
-      in
-      "[" ^ fieldValues ^ "]"
   | TupleC innerTypesC ->
     "["
     ^ (innerTypesC
diff --git a/jscomp/gentype/EmitJs.ml b/jscomp/gentype/EmitJs.ml
index 29ed058782..85e273a6c2 100644
--- a/jscomp/gentype/EmitJs.ml
+++ b/jscomp/gentype/EmitJs.ml
@@ -101,7 +101,7 @@ let typeNameIsInterface ~(exportTypeMap : CodeItem.exportTypeMap)
     ~(exportTypeMapFromOtherFiles : CodeItem.exportTypeMap) typeName =
   let typeIsInterface type_ =
     match type_ with
-    | Object _ | Record _ -> true
+    | Object _ -> true
     | _ -> false
   in
   match exportTypeMap |> StringMap.find typeName with
@@ -620,11 +620,11 @@ let propagateAnnotationToSubTypes ~codeItems (typeMap : CodeItem.exportTypeMap)
             type1 |> visit
           | exception Not_found ->
             annotatedSet := !annotatedSet |> StringSet.add typeName)
-      | Array (t, _) -> t |> visit
+      | Array (t, _) | Dict t -> t |> visit
       | Function {argTypes; retType} ->
         argTypes |> List.iter (fun {aType} -> visit aType);
         retType |> visit
-      | GroupOfLabeledArgs fields | Object (_, fields) | Record fields ->
+      | GroupOfLabeledArgs fields | Object (_, fields) ->
         fields |> List.iter (fun {type_} -> type_ |> visit)
       | Option t | Null t | Nullable t | Promise t -> t |> visit
       | Tuple innerTypes -> innerTypes |> List.iter visit
diff --git a/jscomp/gentype/EmitType.ml b/jscomp/gentype/EmitType.ml
index 4c48468b35..c24f110b2e 100644
--- a/jscomp/gentype/EmitType.ml
+++ b/jscomp/gentype/EmitType.ml
@@ -95,6 +95,10 @@ let rec renderType ~(config : Config.t) ?(indent = None) ~typeNameIsInterface
       arrayName ^ "<"
       ^ (t |> renderType ~config ~indent ~typeNameIsInterface ~inFunType)
       ^ ">"
+  | Dict type_ ->
+    "{[id: string]: "
+    ^ (type_ |> renderType ~config ~indent ~typeNameIsInterface ~inFunType)
+    ^ "}"
   | Function
       {argTypes = [{aType = Object (closedFlag, fields)}]; retType; typeVars}
     when retType |> isTypeFunctionComponent ~fields ->
@@ -116,7 +120,7 @@ let rec renderType ~(config : Config.t) ?(indent = None) ~typeNameIsInterface
   | Function {argTypes; retType; typeVars} ->
     renderFunType ~config ~indent ~inFunType ~typeNameIsInterface ~typeVars
       argTypes retType
-  | GroupOfLabeledArgs fields | Object (_, fields) | Record fields ->
+  | GroupOfLabeledArgs fields | Object (_, fields) ->
     let indent1 = fields |> Indent.heuristicFields ~indent in
     fields
     |> renderFields ~config ~indent:indent1 ~inFunType ~typeNameIsInterface
@@ -136,7 +140,7 @@ let rec renderType ~(config : Config.t) ?(indent = None) ~typeNameIsInterface
     "(null | "
     ^ (type_ |> renderType ~config ~indent ~typeNameIsInterface ~inFunType)
     ^ ")"
-  | Nullable type_ | Option type_ ->
+  | Nullable type_ ->
     let useParens x =
       match type_ with
       | Function _ | Variant _ -> EmitText.parens [x]
@@ -146,6 +150,16 @@ let rec renderType ~(config : Config.t) ?(indent = None) ~typeNameIsInterface
     ^ useParens
         (type_ |> renderType ~config ~indent ~typeNameIsInterface ~inFunType)
     ^ ")"
+  | Option type_ ->
+    let useParens x =
+      match type_ with
+      | Function _ | Variant _ -> EmitText.parens [x]
+      | _ -> x
+    in
+    "(undefined | "
+    ^ useParens
+        (type_ |> renderType ~config ~indent ~typeNameIsInterface ~inFunType)
+    ^ ")"
   | Promise type_ ->
     "Promise" ^ "<"
     ^ (type_ |> renderType ~config ~indent ~typeNameIsInterface ~inFunType)
diff --git a/jscomp/gentype/GenTypeCommon.ml b/jscomp/gentype/GenTypeCommon.ml
index ba8f357e33..b65dc1917a 100644
--- a/jscomp/gentype/GenTypeCommon.ml
+++ b/jscomp/gentype/GenTypeCommon.ml
@@ -59,6 +59,7 @@ type closedFlag = Open | Closed
 
 type type_ =
   | Array of type_ * mutable_
+  | Dict of type_
   | Function of function_
   | GroupOfLabeledArgs of fields
   | Ident of ident
@@ -67,7 +68,6 @@ type type_ =
   | Object of closedFlag * fields
   | Option of type_
   | Promise of type_
-  | Record of fields
   | Tuple of type_ list
   | TypeVar of string
   | Variant of variant
@@ -108,6 +108,7 @@ and payload = {case: case; inlineRecord: bool; numArgs: int; t: type_}
 let typeIsObject type_ =
   match type_ with
   | Array _ -> true
+  | Dict _ -> true
   | Function _ -> false
   | GroupOfLabeledArgs _ -> false
   | Ident _ -> false
@@ -116,7 +117,6 @@ let typeIsObject type_ =
   | Object _ -> true
   | Option _ -> false
   | Promise _ -> true
-  | Record _ -> true
   | Tuple _ -> true
   | TypeVar _ -> false
   | Variant _ -> false
@@ -210,11 +210,17 @@ let sanitizeTypeName name =
        | '\'' -> '_'
        | c -> c)
 let unknown = ident "unknown"
+let bigintT = ident "BigInt"
 let booleanT = ident "boolean"
 let dateT = ident "Date"
+let mapT (x,y) = ident ~typeArgs:[x;y] "Map"
 let numberT = ident "number"
+let regexpT = ident "RegExp"
+let setT (x) = ident ~typeArgs:[x] "Set"
 let stringT = ident "string"
 let unitT = ident "void"
+let weakmapT (x,y) = ident ~typeArgs:[x;y] "WeakMap"
+let weaksetT (x) = ident ~typeArgs:[x] "WeakSet"
 let int64T = Tuple [numberT; numberT]
 
 module NodeFilename = struct
diff --git a/jscomp/gentype/TranslateSignatureFromTypes.ml b/jscomp/gentype/TranslateSignatureFromTypes.ml
index 9dff024d03..79a32969dc 100644
--- a/jscomp/gentype/TranslateSignatureFromTypes.ml
+++ b/jscomp/gentype/TranslateSignatureFromTypes.ml
@@ -12,8 +12,9 @@ let translateTypeDeclarationFromTypes ~config ~outputFileRelative ~resolver
     Log_.item "Translate Types.type_declaration %s\n" typeName;
   let declarationKind =
     match type_kind with
-    | Type_record (labelDeclarations, _) ->
-      TranslateTypeDeclarations.RecordDeclarationFromTypes labelDeclarations
+    | Type_record (labelDeclarations, recordRepresentation) ->
+      TranslateTypeDeclarations.RecordDeclarationFromTypes
+        (labelDeclarations, recordRepresentation)
     | Type_variant constructorDeclarations
       when not
              (TranslateTypeDeclarations.hasSomeGADTLeaf constructorDeclarations)
diff --git a/jscomp/gentype/TranslateTypeDeclarations.ml b/jscomp/gentype/TranslateTypeDeclarations.ml
index 62213f8205..a8229d876d 100644
--- a/jscomp/gentype/TranslateTypeDeclarations.ml
+++ b/jscomp/gentype/TranslateTypeDeclarations.ml
@@ -1,7 +1,8 @@
 open GenTypeCommon
 
 type declarationKind =
-  | RecordDeclarationFromTypes of Types.label_declaration list
+  | RecordDeclarationFromTypes of
+      Types.label_declaration list * Types.record_representation
   | GeneralDeclaration of Typedtree.core_type option
   | GeneralDeclarationFromTypes of Types.type_expr option
       (** As the above, but from Types not Typedtree *)
@@ -78,7 +79,12 @@ let traslateDeclarationKind ~config ~loc ~outputFileRelative ~resolver
     in
     {CodeItem.importTypes; exportFromTypeDeclaration}
   in
-  let translateLabelDeclarations labelDeclarations =
+  let translateLabelDeclarations ~recordRepresentation labelDeclarations =
+    let isOptional l =
+      match recordRepresentation with
+      | Types.Record_optional_labels lbls -> List.mem l lbls
+      | _ -> false
+    in
     let fieldTranslations =
       labelDeclarations
       |> List.map (fun {Types.ld_id; ld_mutable; ld_type; ld_attributes} ->
@@ -111,7 +117,7 @@ let traslateDeclarationKind ~config ~loc ~outputFileRelative ~resolver
            ->
              let optional, type1 =
                match type_ with
-               | Option type1 -> (Optional, type1)
+               | Option type1 when isOptional nameRE -> (Optional, type1)
                | _ -> (Mandatory, type_)
              in
              {mutable_; nameJS; nameRE; optional; type_ = type1})
@@ -196,9 +202,10 @@ let traslateDeclarationKind ~config ~loc ~outputFileRelative ~resolver
     in
     {translation with type_} |> handleGeneralDeclaration
     |> returnTypeDeclaration
-  | RecordDeclarationFromTypes labelDeclarations, None ->
+  | RecordDeclarationFromTypes (labelDeclarations, recordRepresentation), None
+    ->
     let {TranslateTypeExprFromTypes.dependencies; type_} =
-      labelDeclarations |> translateLabelDeclarations
+      labelDeclarations |> translateLabelDeclarations ~recordRepresentation
     in
     let importTypes =
       dependencies
@@ -227,7 +234,11 @@ let traslateDeclarationKind ~config ~loc ~outputFileRelative ~resolver
                  |> TranslateTypeExprFromTypes.translateTypeExprsFromTypes
                       ~config ~typeEnv
                | Cstr_record labelDeclarations ->
-                 [labelDeclarations |> translateLabelDeclarations]
+                 [
+                   labelDeclarations
+                   |> translateLabelDeclarations
+                        ~recordRepresentation:Types.Record_regular;
+                 ]
              in
              let inlineRecord =
                match constructorArgs with
@@ -342,8 +353,8 @@ let translateTypeDeclaration ~config ~outputFileRelative ~recursive ~resolver
   in
   let declarationKind =
     match typ_type.type_kind with
-    | Type_record (labelDeclarations, _) ->
-      RecordDeclarationFromTypes labelDeclarations
+    | Type_record (labelDeclarations, recordRepresentation) ->
+      RecordDeclarationFromTypes (labelDeclarations, recordRepresentation)
     | Type_variant constructorDeclarations ->
       VariantDeclarationFromTypes constructorDeclarations
     | Type_abstract -> GeneralDeclaration typ_manifest
diff --git a/jscomp/gentype/TranslateTypeExprFromTypes.ml b/jscomp/gentype/TranslateTypeExprFromTypes.ml
index 573c5ed996..7d0b966474 100644
--- a/jscomp/gentype/TranslateTypeExprFromTypes.ml
+++ b/jscomp/gentype/TranslateTypeExprFromTypes.ml
@@ -80,7 +80,34 @@ let translateConstr ~config ~paramsTranslation ~(path : Path.t) ~typeEnv =
       | ["Js"; ("String" | "String2"); "t"] ),
       [] ) ->
     {dependencies = []; type_ = stringT}
-  | ["Js"; "Date"; "t"], [] -> {dependencies = []; type_ = dateT}
+  | (["Js"; "Types"; "bigint_val"] | ["BigInt"; "t"]), [] ->
+    {dependencies = []; type_ = bigintT}
+  | (["Js"; "Date"; "t"] | ["Date"; "t"]), [] ->
+    {dependencies = []; type_ = dateT}
+  | ["Map"; "t"], [paramTranslation1; paramTranslation2] ->
+    {
+      dependencies =
+        paramTranslation1.dependencies @ paramTranslation2.dependencies;
+      type_ = mapT (paramTranslation1.type_, paramTranslation2.type_);
+    }
+  | ["WeakMap"; "t"], [paramTranslation1; paramTranslation2] ->
+    {
+      dependencies =
+        paramTranslation1.dependencies @ paramTranslation2.dependencies;
+      type_ = weakmapT (paramTranslation1.type_, paramTranslation2.type_);
+    }
+  | ["Set"; "t"], [paramTranslation] ->
+    {
+      dependencies = paramTranslation.dependencies;
+      type_ = setT paramTranslation.type_;
+    }
+  | ["WeakSet"; "t"], [paramTranslation] ->
+    {
+      dependencies = paramTranslation.dependencies;
+      type_ = weaksetT paramTranslation.type_;
+    }
+  | (["Js"; "Re"; "t"] | ["RegExp"; "t"]), [] ->
+    {dependencies = []; type_ = regexpT}
   | (["FB"; "unit"] | ["unit"]), [] -> {dependencies = []; type_ = unitT}
   | ( (["FB"; "array"] | ["array"] | ["Js"; ("Array" | "Array2"); "t"]),
       [paramTranslation] ) ->
@@ -201,16 +228,24 @@ let translateConstr ~config ~paramsTranslation ~(path : Path.t) ~typeEnv =
     {dependencies = []; type_ = EmitType.typeReactElement}
   | (["FB"; "option"] | ["option"]), [paramTranslation] ->
     {paramTranslation with type_ = Option paramTranslation.type_}
-  | (["Js"; "Null"; "t"] | ["Js"; "null"]), [paramTranslation] ->
+  | ( (["Js"; "Undefined"; "t"] | ["Undefined"; "t"] | ["Js"; "undefined"]),
+      [paramTranslation] ) ->
+    {paramTranslation with type_ = Option paramTranslation.type_}
+  | (["Js"; "Null"; "t"] | ["Null"; "t"] | ["Js"; "null"]), [paramTranslation]
+    ->
     {paramTranslation with type_ = Null paramTranslation.type_}
   | ( ( ["Js"; "Nullable"; "t"]
+      | ["Nullable"; "t"]
       | ["Js"; "nullable"]
       | ["Js"; "Null_undefined"; "t"]
       | ["Js"; "null_undefined"] ),
       [paramTranslation] ) ->
     {paramTranslation with type_ = Nullable paramTranslation.type_}
-  | (["Js"; "Promise"; "t"] | ["promise"]), [paramTranslation] ->
+  | ( (["Js"; "Promise"; "t"] | ["Promise"; "t"] | ["promise"]),
+      [paramTranslation] ) ->
     {paramTranslation with type_ = Promise paramTranslation.type_}
+  | (["Js"; "Dict"; "t"] | ["Dict"; "t"]), [paramTranslation] ->
+    {paramTranslation with type_ = Dict paramTranslation.type_}
   | ["function$"], [arg; _arity] ->
     {
       dependencies = arg.dependencies;
diff --git a/jscomp/gentype/TypeVars.ml b/jscomp/gentype/TypeVars.ml
index 05a9d3a1fe..780ea36dd1 100644
--- a/jscomp/gentype/TypeVars.ml
+++ b/jscomp/gentype/TypeVars.ml
@@ -30,6 +30,7 @@ let extractFromCoreType typeParams =
 let rec substitute ~f type0 =
   match type0 with
   | Array (t, arrayKind) -> Array (t |> substitute ~f, arrayKind)
+  | Dict type_ -> Dict (type_ |> substitute ~f)
   | Function function_ ->
     Function
       {
@@ -57,11 +58,6 @@ let rec substitute ~f type0 =
                {field with type_ = field.type_ |> substitute ~f}) )
   | Option type_ -> Option (type_ |> substitute ~f)
   | Promise type_ -> Promise (type_ |> substitute ~f)
-  | Record fields ->
-    Record
-      (fields
-      |> List.map (fun field ->
-             {field with type_ = field.type_ |> substitute ~f}))
   | Tuple innerTypes -> Tuple (innerTypes |> List.map (substitute ~f))
   | TypeVar s -> (
     match f s with
@@ -84,7 +80,7 @@ let rec free_ type0 : StringSet.t =
     StringSet.diff
       ((argTypes |> freeOfList_) +++ (retType |> free_))
       (typeVars |> StringSet.of_list)
-  | GroupOfLabeledArgs fields | Object (_, fields) | Record fields ->
+  | GroupOfLabeledArgs fields | Object (_, fields) ->
     fields
     |> List.fold_left
          (fun s {type_} -> StringSet.union s (type_ |> free_))
@@ -94,7 +90,7 @@ let rec free_ type0 : StringSet.t =
     |> List.fold_left
          (fun s typeArg -> StringSet.union s (typeArg |> free_))
          StringSet.empty
-  | Null type_ | Nullable type_ -> type_ |> free_
+  | Dict type_ | Null type_ | Nullable type_ -> type_ |> free_
   | Option type_ | Promise type_ -> type_ |> free_
   | Tuple innerTypes ->
     innerTypes
diff --git a/jscomp/gentype_tests/typescript-react-example/src/BigInt.bs.js b/jscomp/gentype_tests/typescript-react-example/src/BigInt.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/BigInt.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/BigInt.res b/jscomp/gentype_tests/typescript-react-example/src/BigInt.res
new file mode 100644
index 0000000000..2a3956d058
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/BigInt.res
@@ -0,0 +1 @@
+type t = Js.Types.bigint_val
\ No newline at end of file
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Core.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Core.bs.js
new file mode 100644
index 0000000000..0864298fc2
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Core.bs.js
@@ -0,0 +1,133 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+
+import * as CoreGen from "./Core.gen";
+
+function null0(x) {
+  return x;
+}
+
+function null1(x) {
+  return x;
+}
+
+function nullable0(x) {
+  return x;
+}
+
+function nullable1(x) {
+  return x;
+}
+
+function undefined0(x) {
+  return x;
+}
+
+function undefined1(x) {
+  return x;
+}
+
+function dict0(x) {
+  return x;
+}
+
+function dict1(x) {
+  return x;
+}
+
+function promise0(x) {
+  return x;
+}
+
+function promise1(x) {
+  return x;
+}
+
+function date0(x) {
+  return x;
+}
+
+function date1(x) {
+  return x;
+}
+
+function bigint0(x) {
+  return x;
+}
+
+function bigint1(x) {
+  return x;
+}
+
+function regexp0(x) {
+  return x;
+}
+
+function regexp1(x) {
+  return x;
+}
+
+function map1(x) {
+  return x;
+}
+
+function weakmap1(x) {
+  return x;
+}
+
+function set1(x) {
+  return x;
+}
+
+function weakset1(x) {
+  return x;
+}
+
+function option0(x) {
+  return x;
+}
+
+function option1(x) {
+  return x;
+}
+
+function someFunWithNullThenOptionalArgs(prim0, prim1) {
+  return CoreGen.someFunWithNullThenOptionalArgs(prim0, prim1);
+}
+
+function someFunWithNullUndefinedArg(prim0, prim1) {
+  return CoreGen.someFunWithNullUndefinedArg(prim0, prim1);
+}
+
+var $$Map;
+
+var $$Set;
+
+export {
+  null0 ,
+  null1 ,
+  nullable0 ,
+  nullable1 ,
+  undefined0 ,
+  undefined1 ,
+  dict0 ,
+  dict1 ,
+  promise0 ,
+  promise1 ,
+  date0 ,
+  date1 ,
+  bigint0 ,
+  bigint1 ,
+  regexp0 ,
+  regexp1 ,
+  $$Map ,
+  $$Set ,
+  map1 ,
+  weakmap1 ,
+  set1 ,
+  weakset1 ,
+  option0 ,
+  option1 ,
+  someFunWithNullThenOptionalArgs ,
+  someFunWithNullUndefinedArg ,
+}
+/* ./Core.gen Not a pure module */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Core.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/Core.gen.tsx
new file mode 100644
index 0000000000..8a5c6608f9
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Core.gen.tsx
@@ -0,0 +1,86 @@
+/* TypeScript file generated from Core.res by genType. */
+/* eslint-disable import/first */
+
+
+import {someFunWithNullThenOptionalArgs as someFunWithNullThenOptionalArgsNotChecked} from './CoreTS';
+
+import {someFunWithNullUndefinedArg as someFunWithNullUndefinedArgNotChecked} from './CoreTS';
+
+const $$toJS552311971: { [key: string]: any } = {"0": "A"};
+
+const $$toRE552311971: { [key: string]: any } = {"A": 0};
+
+// In case of type error, check the type of 'someFunWithNullThenOptionalArgs' in 'Core.res' and './CoreTS'.
+export const someFunWithNullThenOptionalArgsTypeChecked: (_1:(null | string), _2:(undefined | string)) => string = someFunWithNullThenOptionalArgsNotChecked;
+
+// Export 'someFunWithNullThenOptionalArgs' early to allow circular import from the '.bs.js' file.
+export const someFunWithNullThenOptionalArgs: unknown = someFunWithNullThenOptionalArgsTypeChecked as (_1:(null | string), _2:(undefined | string)) => string;
+
+// In case of type error, check the type of 'someFunWithNullUndefinedArg' in 'Core.res' and './CoreTS'.
+export const someFunWithNullUndefinedArgTypeChecked: (_1:(null | undefined | string), _2:number) => string = someFunWithNullUndefinedArgNotChecked;
+
+// Export 'someFunWithNullUndefinedArg' early to allow circular import from the '.bs.js' file.
+export const someFunWithNullUndefinedArg: unknown = someFunWithNullUndefinedArgTypeChecked as (_1:(null | undefined | string), _2:number) => string;
+
+// tslint:disable-next-line:no-var-requires
+const CoreBS = require('./Core.bs');
+
+// tslint:disable-next-line:interface-over-type-literal
+export type variant = "A" | { tag: "B"; value: string };
+
+// tslint:disable-next-line:interface-over-type-literal
+export type t1 = { readonly x?: string };
+
+// tslint:disable-next-line:interface-over-type-literal
+export type t2 = { readonly x: (undefined | string) };
+
+export const null0: (x:(null | number)) => (null | number) = CoreBS.null0;
+
+export const null1: (x:(null | number)) => (null | number) = CoreBS.null1;
+
+export const nullable0: (x:(null | undefined | number)) => (null | undefined | number) = CoreBS.nullable0;
+
+export const nullable1: (x:(null | undefined | number)) => (null | undefined | number) = CoreBS.nullable1;
+
+export const undefined0: (x:(undefined | number)) => (undefined | number) = CoreBS.undefined0;
+
+export const undefined1: (x:(undefined | number)) => (undefined | number) = CoreBS.undefined1;
+
+export const dict0: (x:{[id: string]: string}) => {[id: string]: string} = CoreBS.dict0;
+
+export const dict1: (x:{[id: string]: string}) => {[id: string]: string} = CoreBS.dict1;
+
+export const promise0: (x:Promise<string>) => Promise<string> = CoreBS.promise0;
+
+export const promise1: (x:Promise<string>) => Promise<string> = CoreBS.promise1;
+
+export const date0: (x:Date) => Date = CoreBS.date0;
+
+export const date1: (x:Date) => Date = CoreBS.date1;
+
+export const bigint0: (x:BigInt) => BigInt = CoreBS.bigint0;
+
+export const bigint1: (x:BigInt) => BigInt = CoreBS.bigint1;
+
+export const regexp0: (x:RegExp) => RegExp = CoreBS.regexp0;
+
+export const regexp1: (x:RegExp) => RegExp = CoreBS.regexp1;
+
+export const map1: (x:Map<string,number>) => Map<string,number> = CoreBS.map1;
+
+export const weakmap1: (x:WeakMap<number[],number>) => WeakMap<number[],number> = CoreBS.weakmap1;
+
+export const set1: (x:Set<string>) => Set<string> = CoreBS.set1;
+
+export const weakset1: (x:WeakSet<number[]>) => WeakSet<number[]> = CoreBS.weakset1;
+
+export const option0: (x:(undefined | string)) => (undefined | string) = CoreBS.option0;
+
+export const option1: (x:(undefined | variant)) => (undefined | variant) = function (Arg1: any) {
+  const result = CoreBS.option1((Arg1 == null ? Arg1 : typeof(Arg1) === 'object'
+    ? {TAG: 0, _0:Arg1.value} as any
+    : $$toRE552311971[Arg1]));
+  return (result == null ? result : typeof(result) === 'object'
+    ? {tag:"B", value:result._0}
+    : $$toJS552311971[result])
+};
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Core.res b/jscomp/gentype_tests/typescript-react-example/src/Core.res
new file mode 100644
index 0000000000..75fc44fe3b
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Core.res
@@ -0,0 +1,88 @@
+@genType
+let null0 = (x: Js.null<int>) => x
+
+@genType
+let null1 = (x: Null.t<int>) => x
+
+@genType
+let nullable0 = (x: Js.nullable<int>) => x
+
+@genType
+let nullable1 = (x: Nullable.t<int>) => x
+
+@genType
+let undefined0 = (x: Js.undefined<int>) => x
+
+@genType
+let undefined1 = (x: Undefined.t<int>) => x
+
+@genType
+let dict0 = (x: Js.Dict.t<string>) => x
+
+@genType
+let dict1 = (x: Dict.t<string>) => x
+
+@genType
+let promise0 = (x: promise<string>) => x
+
+@genType
+let promise1 = (x: Promise.t<string>) => x
+
+@genType
+let date0 = (x: Js.Date.t) => x
+
+@genType
+let date1 = (x: Date.t) => x
+
+@genType
+let bigint0 = (x: Js.Types.bigint_val) => x
+
+@genType
+let bigint1 = (x: BigInt.t) => x
+
+@genType
+let regexp0 = (x: Js.Re.t) => x
+
+@genType
+let regexp1 = (x: RegExp.t) => x
+
+module Map = Map_
+module Set = Set_
+
+@genType
+let map1 = (x: Map.t<string, int>) => x
+
+@genType
+let weakmap1 = (x: WeakMap.t<array<int>, int>) => x
+
+@genType
+let set1 = (x: Set.t<string>) => x
+
+@genType
+let weakset1 = (x: WeakSet.t<array<int>>) => x
+
+type variant = A | B(string)
+
+@genType
+let option0 = (x: option<string>) => x
+
+@genType
+let option1 = (x: option<variant>) => x
+
+@genType
+type t1 = {x?: string}
+
+@genType
+type t2 = {x: Js.undefined<string>}
+
+@genType.import("./CoreTS")
+external someFunWithNullThenOptionalArgs: (
+  Null.t<string> /* Cannot be Nullable.t or option */,
+  option<string> /* Cannot be Null.t or Nullable.t */,
+) => string = "someFunWithNullThenOptionalArgs"
+
+@genType.import("./CoreTS")
+external someFunWithNullUndefinedArg: (
+  Nullable.t<string> /* Can also be Null.t or option as they are subtypes */,
+  int,
+) => string = "someFunWithNullUndefinedArg"
diff --git a/jscomp/gentype_tests/typescript-react-example/src/CoreTS.ts b/jscomp/gentype_tests/typescript-react-example/src/CoreTS.ts
new file mode 100644
index 0000000000..ef25fa71eb
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/CoreTS.ts
@@ -0,0 +1,9 @@
+export declare function someFunWithNullThenOptionalArgs(
+  nullable: null | string,
+  app?: string
+): string;
+
+export declare function someFunWithNullUndefinedArg(
+  nullUndefined: null | undefined | string,
+  other: number
+): string;
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Date.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Date.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Date.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Date.res b/jscomp/gentype_tests/typescript-react-example/src/Date.res
new file mode 100644
index 0000000000..87cfe1b554
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Date.res
@@ -0,0 +1 @@
+type t = Js.Date.t
\ No newline at end of file
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Dict.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Dict.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Dict.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Dict.res b/jscomp/gentype_tests/typescript-react-example/src/Dict.res
new file mode 100644
index 0000000000..444e46d661
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Dict.res
@@ -0,0 +1 @@
+type t<'a> = Js.Dict.t<'a>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/ImportJsValue.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/ImportJsValue.gen.tsx
index e5a3949af4..66fda441c6 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/ImportJsValue.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/ImportJsValue.gen.tsx
@@ -107,7 +107,7 @@ import type {polyType as $$polyType} from './MyMath';
 import type {stringFunction as $$stringFunction} from './MyMath';
 
 // tslint:disable-next-line:interface-over-type-literal
-export type point = { readonly x: number; readonly y?: number };
+export type point = { readonly x: number; readonly y: (undefined | number) };
 
 // tslint:disable-next-line:interface-over-type-literal
 export type numberOrString = $$numberOrString;
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Map_.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Map_.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Map_.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Map_.res b/jscomp/gentype_tests/typescript-react-example/src/Map_.res
new file mode 100644
index 0000000000..9f77470451
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Map_.res
@@ -0,0 +1 @@
+type t<'k, 'v>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Null.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Null.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Null.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Null.res b/jscomp/gentype_tests/typescript-react-example/src/Null.res
new file mode 100644
index 0000000000..fff00a9895
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Null.res
@@ -0,0 +1 @@
+type t<'a> = Js.null<'a>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Nullable.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Nullable.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Nullable.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Nullable.res b/jscomp/gentype_tests/typescript-react-example/src/Nullable.res
new file mode 100644
index 0000000000..56a19212ce
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Nullable.res
@@ -0,0 +1 @@
+type t<'a> = Js.nullable<'a>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Object.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/Object.gen.tsx
index 061e2ae084..31cd17915e 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/Object.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/Object.gen.tsx
@@ -6,4 +6,4 @@
 export type someType = { readonly crop?: string; readonly "fp-z"?: string };
 
 // tslint:disable-next-line:interface-over-type-literal
-export type someType2 = { readonly crop?: string; readonly "fp-z"?: string };
+export type someType2 = { readonly crop: (undefined | string); readonly "fp-z": (undefined | string) };
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Promise.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Promise.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Promise.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Promise.res b/jscomp/gentype_tests/typescript-react-example/src/Promise.res
new file mode 100644
index 0000000000..4e149cfece
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Promise.res
@@ -0,0 +1 @@
+type t<'a> = promise<'a>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Records.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/Records.gen.tsx
index 31ef5bb28a..65e582ba1a 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/Records.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/Records.gen.tsx
@@ -16,21 +16,21 @@ import type {list} from '../src/shims/RescriptPervasives.shim';
 export type coord = {
   readonly x: number; 
   readonly y: number; 
-  readonly z?: number
+  readonly z: (undefined | number)
 };
 
 // tslint:disable-next-line:interface-over-type-literal
 export type person = {
   readonly name: string; 
   readonly age: number; 
-  readonly address?: string
+  readonly address: (undefined | string)
 };
 
 // tslint:disable-next-line:interface-over-type-literal
 export type business = {
   readonly name: string; 
-  readonly owner?: person; 
-  readonly address?: string
+  readonly owner: (undefined | person); 
+  readonly address: (undefined | string)
 };
 
 // tslint:disable-next-line:interface-over-type-literal
diff --git a/jscomp/gentype_tests/typescript-react-example/src/RegExp.bs.js b/jscomp/gentype_tests/typescript-react-example/src/RegExp.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/RegExp.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/RegExp.res b/jscomp/gentype_tests/typescript-react-example/src/RegExp.res
new file mode 100644
index 0000000000..afd4e13e6f
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/RegExp.res
@@ -0,0 +1 @@
+type t = Js.Re.t
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Set_.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Set_.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Set_.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Set_.res b/jscomp/gentype_tests/typescript-react-example/src/Set_.res
new file mode 100644
index 0000000000..cfacdaa67e
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Set_.res
@@ -0,0 +1 @@
+type t<'a>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/TestImmutableArray.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/TestImmutableArray.gen.tsx
index acb9f91d84..6ea14f6e0f 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/TestImmutableArray.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/TestImmutableArray.gen.tsx
@@ -6,4 +6,4 @@
 import * as TestImmutableArrayBS__Es6Import from './TestImmutableArray.bs';
 const TestImmutableArrayBS: any = TestImmutableArrayBS__Es6Import;
 
-export const testImmutableArrayGet: <T1>(arr:ReadonlyArray<T1>) => (null | undefined | T1) = TestImmutableArrayBS.testImmutableArrayGet;
+export const testImmutableArrayGet: <T1>(arr:ReadonlyArray<T1>) => (undefined | T1) = TestImmutableArrayBS.testImmutableArrayGet;
diff --git a/jscomp/gentype_tests/typescript-react-example/src/TestPromise.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/TestPromise.gen.tsx
index 03444652a6..fb57d54ad6 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/TestPromise.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/TestPromise.gen.tsx
@@ -21,7 +21,7 @@ export type toPayload = { readonly result: string };
 
 export const convert: (_1:Promise<fromPayload>) => Promise<toPayload> = TestPromiseBS.convert;
 
-export const barx: (_1:{ readonly x?: Promise<(null | undefined | string)> }, _2:void) => boolean = function (Arg1: any, Arg2: any) {
-  const result = Curry._2(TestPromiseBS.barx, (Arg1.x == null ? undefined : Arg1.x.then(function _element($promise: any) { return ($promise == null ? undefined : $promise)})), Arg2);
+export const barx: (_1:{ readonly x?: Promise<(undefined | string)> }, _2:void) => boolean = function (Arg1: any, Arg2: any) {
+  const result = Curry._2(TestPromiseBS.barx, Arg1.x, Arg2);
   return result
 };
diff --git a/jscomp/gentype_tests/typescript-react-example/src/TransitiveType1.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/TransitiveType1.gen.tsx
index c8ae43db6c..7d343e4543 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/TransitiveType1.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/TransitiveType1.gen.tsx
@@ -10,12 +10,6 @@ import type {t2Alias as TransitiveType2_t2Alias} from './TransitiveType2.gen';
 
 import type {t2 as TransitiveType2_t2} from './TransitiveType2.gen';
 
-export const convert: (x:TransitiveType2_t2) => TransitiveType2_t2 = function (Arg1: any) {
-  const result = TransitiveType1BS.convert((Arg1 == null ? undefined : Arg1));
-  return result
-};
+export const convert: (x:TransitiveType2_t2) => TransitiveType2_t2 = TransitiveType1BS.convert;
 
-export const convertAlias: (x:TransitiveType2_t2Alias) => TransitiveType2_t2Alias = function (Arg1: any) {
-  const result = TransitiveType1BS.convertAlias((Arg1 == null ? undefined : Arg1));
-  return result
-};
+export const convertAlias: (x:TransitiveType2_t2Alias) => TransitiveType2_t2Alias = TransitiveType1BS.convertAlias;
diff --git a/jscomp/gentype_tests/typescript-react-example/src/TransitiveType2.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/TransitiveType2.gen.tsx
index 4d14e4870f..0ccf1b920c 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/TransitiveType2.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/TransitiveType2.gen.tsx
@@ -5,7 +5,7 @@
 import type {t3 as TransitiveType3_t3} from './TransitiveType3.gen';
 
 // tslint:disable-next-line:interface-over-type-literal
-export type t2 = (null | undefined | TransitiveType3_t3);
+export type t2 = (undefined | TransitiveType3_t3);
 
 // tslint:disable-next-line:interface-over-type-literal
 export type t2Alias = t2;
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Undefined.bs.js b/jscomp/gentype_tests/typescript-react-example/src/Undefined.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Undefined.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Undefined.res b/jscomp/gentype_tests/typescript-react-example/src/Undefined.res
new file mode 100644
index 0000000000..5c2e5dc3de
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/Undefined.res
@@ -0,0 +1 @@
+type t<'a> = Js.undefined<'a>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/Variants.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/Variants.gen.tsx
index 4ed7a98575..7244a0abe3 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/Variants.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/Variants.gen.tsx
@@ -112,7 +112,7 @@ export const id2: (x:x2) => x2 = function (Arg1: any) {
   return $$toJS1061900109[result]
 };
 
-export const polyWithOpt: (foo:string) => (null | undefined | (
+export const polyWithOpt: (foo:string) => (undefined | (
     { NAME: "One"; VAL: string }
   | { NAME: "Two"; VAL: number })) = VariantsBS.polyWithOpt;
 
diff --git a/jscomp/gentype_tests/typescript-react-example/src/VariantsWithPayload.res b/jscomp/gentype_tests/typescript-react-example/src/VariantsWithPayload.res
index 50233719d7..cafe5347a7 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/VariantsWithPayload.res
+++ b/jscomp/gentype_tests/typescript-react-example/src/VariantsWithPayload.res
@@ -1,6 +1,6 @@
 type payload = {
   x: int,
-  y: option<string>,
+  y?: string,
 }
 
 type withPayload = [
diff --git a/jscomp/gentype_tests/typescript-react-example/src/WeakMap.bs.js b/jscomp/gentype_tests/typescript-react-example/src/WeakMap.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/WeakMap.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/WeakMap.res b/jscomp/gentype_tests/typescript-react-example/src/WeakMap.res
new file mode 100644
index 0000000000..9f77470451
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/WeakMap.res
@@ -0,0 +1 @@
+type t<'k, 'v>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/WeakSet.bs.js b/jscomp/gentype_tests/typescript-react-example/src/WeakSet.bs.js
new file mode 100644
index 0000000000..d856702bfe
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/WeakSet.bs.js
@@ -0,0 +1,2 @@
+// Generated by ReScript, PLEASE EDIT WITH CARE
+/* This output is empty. Its source's type definitions, externals and/or unused code got optimized away. */
diff --git a/jscomp/gentype_tests/typescript-react-example/src/WeakSet.res b/jscomp/gentype_tests/typescript-react-example/src/WeakSet.res
new file mode 100644
index 0000000000..cfacdaa67e
--- /dev/null
+++ b/jscomp/gentype_tests/typescript-react-example/src/WeakSet.res
@@ -0,0 +1 @@
+type t<'a>
diff --git a/jscomp/gentype_tests/typescript-react-example/src/nested/Tuples.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/nested/Tuples.gen.tsx
index d5fb83f805..67223c651e 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/nested/Tuples.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/nested/Tuples.gen.tsx
@@ -11,7 +11,7 @@ import * as TuplesBS__Es6Import from './Tuples.bs';
 const TuplesBS: any = TuplesBS__Es6Import;
 
 // tslint:disable-next-line:interface-over-type-literal
-export type coord = [number, number, (null | undefined | number)];
+export type coord = [number, number, (undefined | number)];
 
 // tslint:disable-next-line:interface-over-type-literal
 export type coord2 = [number, number, (null | undefined | number)];
@@ -24,21 +24,15 @@ export type couple = [person, person];
 
 export const testTuple: (param:[number, number]) => number = TuplesBS.testTuple;
 
-export const origin: [number, number, (null | undefined | number)] = TuplesBS.origin;
+export const origin: [number, number, (undefined | number)] = TuplesBS.origin;
 
-export const computeArea: (param:[number, number, (null | undefined | number)]) => number = function (Arg1: any) {
-  const result = TuplesBS.computeArea([Arg1[0], Arg1[1], (Arg1[2] == null ? undefined : Arg1[2])]);
-  return result
-};
+export const computeArea: (param:[number, number, (undefined | number)]) => number = TuplesBS.computeArea;
 
-export const computeAreaWithIdent: (param:coord) => number = function (Arg1: any) {
-  const result = TuplesBS.computeAreaWithIdent([Arg1[0], Arg1[1], (Arg1[2] == null ? undefined : Arg1[2])]);
-  return result
-};
+export const computeAreaWithIdent: (param:coord) => number = TuplesBS.computeAreaWithIdent;
 
 export const computeAreaNoConverters: (param:[number, number]) => number = TuplesBS.computeAreaNoConverters;
 
-export const coord2d: <T1,T2,T3>(x:T1, y:T2) => [T1, T2, (null | undefined | T3)] = function <T1,T2,T3>(Arg1: any, Arg2: any) {
+export const coord2d: <T1,T2,T3>(x:T1, y:T2) => [T1, T2, (undefined | T3)] = function <T1,T2,T3>(Arg1: any, Arg2: any) {
   const result = Curry._2(TuplesBS.coord2d, Arg1, Arg2);
   return result
 };
diff --git a/jscomp/gentype_tests/typescript-react-example/src/nested/Types.gen.tsx b/jscomp/gentype_tests/typescript-react-example/src/nested/Types.gen.tsx
index 1c7fff0692..3195af09e6 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/nested/Types.gen.tsx
+++ b/jscomp/gentype_tests/typescript-react-example/src/nested/Types.gen.tsx
@@ -10,8 +10,6 @@ const Curry: any = Curry__Es6Import;
 import * as TypesBS__Es6Import from './Types.bs';
 const TypesBS: any = TypesBS__Es6Import;
 
-import type {Dict_t as Js_Dict_t} from '../../src/shims/Js.shim';
-
 import type {Json_t as Js_Json_t} from '../../src/shims/Js.shim';
 
 import type {M_t__ as TypeNameSanitize_M_t__} from '../../src/TypeNameSanitize.gen';
@@ -57,7 +55,7 @@ export type twice<a> = [a, a];
 export type genTypeMispelled = number;
 
 // tslint:disable-next-line:interface-over-type-literal
-export type dictString = Js_Dict_t<string>;
+export type dictString = {[id: string]: string};
 
 // tslint:disable-next-line:interface-over-type-literal
 export type nullOrString = (null | string);
@@ -128,8 +126,8 @@ export const selfRecursiveConverter: (param:selfRecursive) => selfRecursive = Ty
 
 export const mutuallyRecursiveConverter: (param:mutuallyRecursiveA) => mutuallyRecursiveB = TypesBS.mutuallyRecursiveConverter;
 
-export const testFunctionOnOptionsAsArgument: <T1,a>(a:(null | undefined | a), foo:((_1:(null | undefined | a)) => T1)) => T1 = function <T1,a>(Arg1: any, Arg2: any) {
-  const result = Curry._2(TypesBS.testFunctionOnOptionsAsArgument, (Arg1 == null ? undefined : Arg1), Arg2);
+export const testFunctionOnOptionsAsArgument: <T1,a>(a:(undefined | a), foo:((_1:(undefined | a)) => T1)) => T1 = function <T1,a>(Arg1: any, Arg2: any) {
+  const result = Curry._2(TypesBS.testFunctionOnOptionsAsArgument, Arg1, Arg2);
   return result
 };
 
@@ -155,4 +153,4 @@ export const currentTime: Date = TypesBS.currentTime;
 
 export const i64Const: i64B = TypesBS.i64Const;
 
-export const optFunction: (null | undefined | (() => number)) = TypesBS.optFunction;
+export const optFunction: (undefined | (() => number)) = TypesBS.optFunction;
diff --git a/jscomp/gentype_tests/typescript-react-example/src/shims/Js.shim.ts b/jscomp/gentype_tests/typescript-react-example/src/shims/Js.shim.ts
index f89f1b26c4..7c735117fd 100644
--- a/jscomp/gentype_tests/typescript-react-example/src/shims/Js.shim.ts
+++ b/jscomp/gentype_tests/typescript-react-example/src/shims/Js.shim.ts
@@ -1,5 +1,3 @@
-export type Dict_t<T> = {[id: string]: T};
-
 export type Json_t = unknown;
 
 export type t = unknown;